Error happen when show commandtool file

[mmm2006]

I want to show a simple commandlinetool file:
class: CommandLineTool
cwlVersion: v1.0
$namespaces:
sbg: 'https://www.sevenbridges.com/'
id: print_string_tool
baseCommand:

• python3
• printString.py
  inputs:
• id: path
  type: string
  inputBinding:
  position: 0
  outputs:
• id: output
  type: string?
  outputBinding:
  glob: print_string_output.txt
  outputEval: '$(self[0].path)'
  requirements:
• class: InitialWorkDirRequirement
  listing:
  • entryname: printString.py
    entry: |

    -- coding: utf-8 --

    if name == "main":
    input_string = "$(inputs.path)"
    print(input_string)
    writable: false
• class: InlineJavascriptRequirement
  label: print-string-tool
  stdout: print_string_output.txt

But it fails like this:

Uncaught Error: Cannot subscribe to a non-supported event “step.change”.
Supported events are: input.create, input.remove, input.change, input.change.id, io.change.type, output.create, output.remove, output.change.id, argument.create, argument.remove, field.create, field.remove, validate, binding.shellQuote, expression.create, expression.change, expression.serialize, output.metadata.inherit”
at EventHub.guard (EventHub.js?b20a:38)
at EventHub.on (EventHub.js?b20a:12)
at V1CommandLineToolModel.CommandLineToolModel.on (CommandLineToolModel.js?cb43:86)
at Workflow.draw (workflow.js?1ee1:122)
at new Workflow (workflow.js?1ee1:77)
at eval (CwlContainer.vue?e091:152)

I don't know how to fix it?

please help me. Thanks.

my use below version;
cwl-svg@2.0.1
cwlts@1.15.29